It has been proposed in U.S. Pat. No. 3,841,953 to form nonwoven webs of melt-blown fibers using polymer blends, in order to obtain webs having novel properties. A problem with these webs, however, is that the polymer interfaces causes weaknesses in the individual fibers that causes severe fiber breakage and weak points. The web tensile properties reported in this patent are inferior to those of webs made of corresponding single polymer fibers. This is likely done to weak points in the web from incompable polymer blends and the extremely short fibers in the web.
A method for producing bicomponent fibers in a melt-blown process is disclosed in U.S. Pat. No. 4,729,371. The polymeric materials are fed from two conduits which meet at a 180 degree angle. The polymer flowstreams then converge and exit via a third conduit at a 90 degree angle to the two feed conduits. The two feedstreams form a layered flowstream in this third conduit, which bilayered flowstream is fed to a row of side-by-side orifices in a melt-blowing die. The layered polymer melt streams extruded from the orifices are then formed into microfibers by a high air velocity attenuation or a "melt-blown" process. The product formed is used specifically to form a web useful for molding into a filter material. The process disclosed concerns forming two-layer microfibers. There is no disclosure of how to obtain webs with strengths significantly greater than webs formed of the individual materials used to form their moldable web.
U.S. Pat. No. 4,557,972 discloses a sheath-core composite fiber of an allegedly ultrafine denier (less than 0.5 denier). The fibers are formed from a special spinneret for forming large, three-component fibers, with two of the components forming ultrafine included material in a matrix of the third component. Ultrafine fibers are then obtained by selectively removing the matrix (the "sea") material, leaving the included material as fine fibers. This process is complex and cannot practically be used to form non-woven webs. Similar processes are proposed by U.S. Pat. Nos. 4,460,649, 4,627,950 and 4,381,274, which discuss various "islands-in-a-sea" processes for forming multi-component yarns. U.S. Pat. No. 4,117,194 describes a bicomponent textile spun fiber with improved crimp properties.
U.S. Pat. Nos. 3,672,802 and 3,681,189 describe spun fibers allegedly having a large number of layers of two polymer components. The two polymers are fed into a specially designed manifold that repeatedly combines, splits and re-combines a polymer stream(s) to form a somewhat stratified stream of the two distinct polymers. The process is similar to mixing the polymers due to the significant amount of non-linear polymer flow introduced during the repeated splitting and re-combining of the polymer stream(s). However, the splitting and re-combining is done in line with the polymer flow, and the resulting fibers apparently have distinct longitudinal regions of one or the other polymer rather than the substantially non-directional arrangement of the separate polymer regions one would obtain with incomplete batch mixing. However, the polymer layers in the fibers are very indistinct and irregular, and due to the excessively long contact period between the polymers, it would be difficult to handle polymers with significantly different melt viscosities by this process. The fibers produced are textile size, and the layering effect is done to improve certain properties over homogeneous fibers such as dyeability properties, electrification properties, hydrophilic properties or tensile properties. No mention is made of making multi-layer microfiber nonwoven webs having improved modulus.